recursion methon added

factorial.c

@@ -1,16 +1,21 @@
 #include <stdio.h>
-
+unsigned long long factorial(int n) {
+    if (n == 0 || n == 1) 
+        return 1;
+    else
+        return n * factorial(n - 1); 
+}
 int main() {
     int n;
-    unsigned long long factorial = 1;
 
     printf("Enter a number: ");
     scanf("%d", &n);
 
-    for(int i = 1; i <= n; ++i) {
-        factorial *= i+1;
+    if (n < 0) {
+        printf("Factorial is not defined for negative numbers.\n");
+    } else {
+        printf("Factorial of %d = %llu\n", n, factorial(n));
     }
 
-    printf("Factorial of %d = %llu\n", n, factorial);
     return 0;
 }